The present invention relates to a heat exchanger and a method of exchanging heat and, more particularly, to an evaporative heat transfer apparatus comprising a direct evaporative heat exchange section and an indirect evaporative heat exchange section.
Evaporative heat transfer units comprising both direct and indirect heat transfer sections are disclosed in U.S. Pat. No. 5,435,382. This patent discloses a design that allows the collection of the evaporative liquid from the direct evaporative section and then pumping it upwardly to redistribute it over the indirect evaporative section. Two limitations exist with the prior art described in this patent. First, the evaporative fluid must be pumped upwardly from the collection basin located below the direct evaporative section for distribution over the indirect evaporative section. This means the indirect evaporative section must be located in the upper section of the heat exchange apparatus. While this arrangement provides benefits for accessibility of the indirect section after installation, it puts additional requirements on the apparatus structure to support the mass of the indirect section at higher elevations. Secondly, when desiring to maximize the thermal capability per apparatus plan area, the plan area occupied by the indirect heat transfer section subtracts from the plan area of the apparatus available for the vertical flow of the hot discharge air. The total apparatus airflow must then pass through this remaining smaller net discharge plan area. The air moving device size may also be smaller than optimum due to the reduced size of the net discharge plan area. Due to the need for both the indirect heat transfer section plan area and the net discharge plan area to occupy separate portions of the total apparatus plan area, neither area can be made as large as desired.
A combined direct and indirect heat exchange apparatus is disclosed with the direct section located above the indirect section in U.S. Pat. No. 5,724,828. However, there still exists a problem with maintaining consistent and uniform spray water flow over the indirect section. No provision is made to account for the pull in of the evaporative liquid due to the horizontal flow of the inlet air stream. As the air moves into the unit, it pulls the outer edges of the evaporative liquid falling from the bottom of the direct section inwardly causing the effective wetted plan area available for the indirect section to be smaller than the plan area of the direct section overhead. Additionally, since the falling water is not pulled in uniformly over the entire plan area nor is the pull in consistent with varying fan power levels, the resulting water spray over the indirect section is not uniform. This distracts from the optimum performance that could be achieved with uniform distribution of the evaporative liquid over the entire indirect heat transfer section.
U.S. Pat. No. 6,598,862 discloses a combined indirect and direct heat exchange apparatus wherein the indirect section is of smaller plan area than the direct evaporative section located above it. This application teaches that higher performance is achieved by not allowing any airflow through the indirect section and discounts the additive performance effect of this additional evaporative surface. This limits the size and capacity of the indirect section that can be used in a given plan area. As with other prior art designs, performance also suffers due to the inconsistent and non-uniform spray water loading at the top of the indirect evaporative section. Furthermore, this design teaches to accelerate the velocity of the falling evaporative liquid to at least 9.5 feet per second and up to 15 feet per second. The claimed purpose of these higher velocities is to improve the heat transfer coefficient of the falling evaporative liquid film over the outside surface of the coil. What impact, if any, this higher velocity liquid may have is limited to the top surface of the coil only. Once the liquid hits the top surface, the flow energy is dissipated and the flow through the rest of the coil is the same as it would be if the evaporative liquid had an initial velocity of zero.